In order to minimize costs and maximize vibration testing utility, test economics require that, when using an electro-dynamic shaker, that as large a number of test specimens as possible be placed on the shaker. This permits a rapid accumulation of test hours, thereby gaining high statistical confidence for a given test time. However, often frequencies are induced that excite the resonant frequency of some portion of the platen (the vibrating platform) onto which the test specimens are bolted.
Those test specimens attached to the resonating portion of the platen will experience much higher stress than either the test plan requires or that of items attached to a non-resonating portion of the platen. In addition, the location of the resonating portion of the platen changes as different frequency inputs excite different spring-mass systems at their individual natural frequencies.
When, at a certain frequency, one test specimen is subjected to greater vibrational stresses than another, the test may provide erroneous failure data for test and reliability analysis. This problem can defeat the entire purpose of the test and causes confusion as to the correct classification of a vibration caused failure. This problem has existed since a large number of items have been subjected to a wide band of vibrational frequencies for the purpose of life testing.
In the past, to solve the resonant frequency problem, four methods have been used. One solution was to use a constant thickness platen thick enough to prevent resonance in the platen corner for a given band of test frequencies. This solution proved unsatisfactory because the combined weight of the test specimens and the platen weigh more than the armature of the shaker can withstand. To maintain certain accelerations at low frequencies, a high displacement is required of the shaker. As the static load is increased on the armature, it lowers from a neutral position to a position closer to its lower limit of travel. Even if the static load is such that the lower limit safety switch is not activated, there is difficulty with activation at the lower end of the frequency spectrum required by the test program.
In the second method, if the platen cannot be made thick enough, then its mounting must be reduced in size, thereby decreasing the number of specimens to be tested by a single shaker. This would add to either the test time or the number of shakers required.
In another method, the solution is to avoid vibration at those frequencies that cause platen resonance. Since each corner of the platen will have at least one resonant frequency, a minimum of four very narrow bands of frequencies will have to be deleted from the test. However, complex electronic and mechanical systems have many small components that behave as individual spring-mass systems which, in turn, have individual resonant frequencies. Elimination of those frequencies which are in narrow bands of each platen resonance results in removal of some of the critical test frequencies for certain components thereby defeating the purpose of the test.
A final solution is to make the platen from a material with high viscous damping. The system is allowed to go through the various bands of resonance but would not reach high vibration amplitudes. Materials such as high viscous magnesium and certain irons are often used to keep amplitudes low at resonance, but some overstressing of test parts results. Instead of amplitude amplification on the order of six or more, it is usually dampened to two or three, but even at these amplitudes, parts are often overstressed, yielding erroneous test results.
The current design practice is to build a square flat plate platen of constant thickness, bolted onto the top of around shaker head, which causes a corner to resonate. The cantilevered corner behaves as a triangular plate which bends beginning from the point where the outer perimeter of the shaker head contacts the bottom of the platen. The difficulty with cantilevers is that the greater the structural masses extend from the line of bending, the lower the natural frequency. In addition, there exists a direct correlation between the area moment of inertia and the resonant frequency. A constant thickness flat plate platen has both unneeded mass at the outer corners of the vibrating triangle and a small area moment of inertia at the root of the cantilevered triangle where bending begins.
The present invention overcomes the various undesirable consequences which attend the existing equipment and methodology for vibration testing by using an inverted, truncated pyramid; the outer portion of the platen has a very low mass and the section modulus is very high at the line of bending thereby increasing the resonant frequency of the platen while lowering the total weight of the platen, because there is less mass around the perimeter of the platen.
It is an object of this invention to provide equipment and methodology for vibration testing which increases the resonant frequency of the platen.
It is another object of this invention to decrease the total weight of the platen, thereby allowing higher displacements at low frequencies of excitation.
Another object of this invention is to decrease the amount of unneeded mass thereby effecting a cost savings for construction material.
Still another object of this invention therefore, is to provide a system that provides uniform vibration stresses for all specimens subjected to the test.
Another object of this invention is to provide a system for reliability testing at the lower end of the frequency spectrum.
It is another an object of this invention to provide an economic and reliable means for vibration testing of testing multiple devices at the same time, thereby decreasing test time and the number of shakers otherwise required.
Still another object of this invention is to provide a method for testing at all critical test frequencies without elimination of any test frequencies.
It is another object of this invention to provide a system for vibration testing without overstressing of test parts.
Further objects and advantages of this invention will become more apparent in light of the following drawings and description of the preferred embodiment of the invention.